a
Aqueous of solution of \(CoCl_2\) contains \([Co(H_2O)_6]^{2+}\) which is pinkish in colour so option \((d)\) is incorrect. Reduction potential of \(C{o^{3 \oplus }} \to C{o^{2 \oplus }}\) is high so option \((b)\) is incorrect. \(Co^{2+}\) does not oxidises easily to \(Co^{3+}\). It is general case that symmetrical substituted octahedral complexes are less deeper in colour than tetrahedral complexes. So \([CoCl_4]^{-2}\) is deep blue in colour.